1. Field of the Invention
The invention relates to a synchronizing mechanism for correlated seat/backrest motion of an office chair, comprising a base carrier to be placed on a chair column; a seat carrier, which is pivotable about a transverse axis and, at its front end, articulated to the base carrier; a backrest carrier, which is also pivotable about a transverse axis and articulated to the base carrier and which is coupled with the seat carrier such that a backward pivoting motion of the backrest induces a lowering motion of the rear end of the seat carrier; the backrest carrier being pivotably mounted on the base carrier by a cam arrangement that defines a four-bar chain; the cam arrangement being comprised of front and rear cams that are articulated to the base carrier and of the backrest carrier itself as a coupling of the four-bar chain; and the seat carrier, by its rear end, being articulated to the backrest carrier.
2. Background Art
The term “synchronizing mechanism” means structural components in the substructure of an office chair which provide for kinematics that involve a certain coupled motion of the seat relative to the backrest. Placed on a chair column is a base carrier on which, on the one hand, is mounted a seat carrier which is pivotable about a transverse axis and articulated to the base carrier and, on the other hand, a backrest carrier which is pivotable about a transverse axis and articulated to the base carrier. Mounted on the seat carrier is the seat of the office chair, which is as a rule provided with an upholstered seat panel. Conventionally, the backrest carrier extends backwards from the virtual synchronizing mechanism, supporting the backrest of the office chair on an upward prolongation.
The seat carrier and the backrest carrier are jointed such that pivoting the backrest backwards—caused for instance when someone sitting on the chair leans back against the backrest—induces the rear edge of the seat to be lowered. This correlated seat/backrest motion is quite convenient and desirable for orthopedic reasons.
A frequent problem in prior art synchronizing mechanisms resides in the pivoting angle of the backrest which is often rather restricted. Moreover, very often the lowering motion of the seat panel is not sufficiently great even with a comparatively great pivoting angle of the backrest, which is due to the pivotable mounting of the seat and backrest carrier in a single joint on the base carrier.
In an office chair of the generic type known from DE 199 21 153 A1, the above outlined objects are attained by the backrest carrier being mounted on the base carrier by way of a cam arrangement that defines a four-bar chain. The cam arrangement is comprised of two cams that are articulated to the base carrier and of the backrest carrier itself as a coupling of the four-bar chain. The longitudinal axes of the two cams form an acute angle that opens upwards towards the seat.
Mounting the backrest support in the way of a four-bar chain as mentioned above fundamentally helps achieve a great pivoting angle, with the backrest carrier not only rotating about a fixed pivot axis, but making an additional backward tilting motion. This pronounced rotary tilting motion is rendered possible by the cams opening upwards.
Provision is further made for the front end of the seat carrier to be articulated to the base carrier by another cam. In the upright position of the synchronizing mechanism, this cam inclines forwards. The rear end of the seat carrier is articulated to the backrest carrier.
Due to the arrangement of the front cam inclining forwards, the seat, starting from the upright position of the synchronizing mechanism—i.e. the base position—will perform a pronounced elevating motion by its front edge at least at the beginning of the correlated seat/backrest motion. The effect is that the user, when actuating the synchronizing mechanism by pressing the backrest backwards, must work against her/his own weight on the seat.
Individually varying preferences involve that some individuals will consider this effect a drawback. This also implies the pronounced elevating motion mentioned above of the seat carrier and thus of the seat of an office chair at the beginning of the synchronous seat/backrest motion.
Finally, the articulated mounting of the seat carrier by an overall of three pairs of cams on either side of the longitudinal center plane of the seat is a comparatively complicated construction.
DE 198 10 768 A1 teaches a synchronizing mechanism in which the backrest carrier is articulated to the base carrier in a single point, fulfilling a pure pivoting motion. At its rear end the seat carrier of this synchronizing mechanism is coupled by a point of articulation with the backrest carrier. At its front end it is cam-controlled in an oblong hole that ascends from front to back. The overall construction of this backrest/seat carrier articulation is simple, but its kinematics fail to comply with practice requirements. In particular the ascending oblong hole for cam-controlled mounting of the front end of the seat carrier causes the elevating motion of the seat front edge, which has been mentioned in connection with the construction according to DE 199 21 153 A1.